Tuberculosis (TB) as a disease continues to result in millions of deaths each year. Inadequate use of chemotherapy has led to an increasing number of drug resistant cases. This situation is likely to worsen with the emergence of extremely resistant strains to all currently known drugs (Van Rie and Enarson, 2006). The internationally recommended TB control strategy, also referred to as directly observed short-course chemotherapy (DOTS), relies on a combination of five antibacterial agents to be taken for a protracted period of more than six months (http://www.who.int/tb/dots/en/). With the use of a mathematical model, taking into consideration treatment duration and TB dynamics, benefits of reduced treatment length were predicted to be substantial and likely to greatly contribute to a reduced global TB burden (Salomon et al., 2006).
Current chemotherapy consists of compounds that directly target Mycobacterium tuberculosis bacillus, either by neutralizing general information pathways and critical processes such as RNA polymerization and protein synthesis inhibition or by interfering with mycobacterial specific cell envelope synthesis. The most widely used dedicated anti-tubercular drugs isoniazid, ethionamide and pyrazinamide are pro-drugs that first require activation. As active forms, they demonstrate inhibitory activity on a wide range of mycobacterial targets, which have not yet been fully characterized. As for other chronic infectious diseases like human immunodeficiency virus, a multi-therapy approach, including drugs that target a wide range of critical features of M. tuberculosis, proved to be the most successful strategy to date. It is, thus, likely that a combination of current drug inhibitors, having different mechanisms of action against M. tuberculosis, will be the solution for the control of the disease.
The most challenging approaches for discovering new anti-TB drugs rely on screening for active compounds that target critical features essential for the survival of the bacillus. Although there is still a lack of understanding of the biological mechanisms behind tubercle bacillus persistence, i.e. the location and state of latent bacteria, in humans, M. tuberculosis is thought to reside in primary granulomas under hypoxic conditions (Lenaerts et al., 2007) as well as to hide within various types of cells (Houben et al., 2006; Neyrolles et al., 2006). The bacillus mainly localizes inside phagocytic cells, such as macrophages and dendritic cells, and it has clearly been established that the tubercle bacillus adopts a different phenotype in the host macrophage's phagosome compared to growth in extracellular conditions (Rohde et al., 2007; Schnappinger et al., 2003). Upon infection, an inflammatory response is induced, thereby initiating recruitment of T lymphocytes that release interleukins and cytokines, which in turn activate the infected macrophages to enable the destruction of the pathogen. Upon the appropriate trigger, the host macrophage is, thus, able to eliminate the invading bacillus. This is further supported by the fact that of the people that inhale M. tuberculosis, more than 95% percent do not develop the disease, suggesting that the human host response is sufficient in most cases to thwart M. tuberculosis induced pathogenesis. This gives rise to the hypothesis that small molecular compounds could mimic the immune cell response signals and induce the host cells to clear the mycobacteria.
Accordingly, it was an object of the present invention to develop a phenotypic cell-based assay suitable for high throughput screening that allows for the search of compounds that would prevent M. tuberculosis multiplication inside the host macrophage.
Up to now, this type of investigation of the tubercle bacillus growth within host cells relied on colony forming units (CFUs) determination after host cell lysis followed by serial dilutions and a 3-week incubation period required for bacterial growth on agar plates. Luciferase-expressing mycobacteria have been shown to be efficient in reducing the experiment duration, although cell lysis and luciferin substrate addition steps are still required (Arain et al., 1996). Also, these types of experiments are not easily amenable to large scale screening.
It was another object of the present invention to identify compounds effective against bacterial infections, in particular compounds that would prevent M. tuberculosis multiplication inside the host macrophage.